Optical sources are used in a variety of optical sensing applications. Although the number of sensing applications they are used in prevents listing them all herein, one application of optical sources is acoustic sensing using towed, fixed and hull mounted arrays. Another application is terrestrial based systems for the detection of ground borne sources of vibration and motion in particular those associated with oil and gas exploration. Still other applications are the use of sensors in pipelines and the monitoring of secure facilities.
Previously known optical sources used in optical fiber sensing are typically either high coherence single wavelength devices or low coherence broadband devices. The selection of an optical source for a particular application depends on the type of optical sensor used, as well as the system noise and detection requirements specific to the application.
Systems utilizing optical sources will generally include other optical devices as well. Such devices include, but are not limited to, fiber Bragg gratings, optical circulators, pump lasers, doped fibers, doped waveguides, amplified spontaneous emission sources, super luminescent diodes, and optical isolators. A fiber Bragg grating (FBG) is a periodic or aperiodic perturbation of the index of refraction in the core of an optical waveguide. The period of the index perturbation determines the wavelength or wavelength band the grating will affect. Depending on the depth of the index perturbation the FBG is capable of reflecting all or a portion of the desired optical wavelength while allowing other wavelengths to pass through unaffected. An optical circulator is typically a three port device that is capable of rotating the inputs and outputs of the device in a cyclic manner. That is, a signal input on port 1 will exit the device on port 2, a signal input on port 2 will exit on port 3, and signals are prevented from passing from the first port to the third port, and from the second port to the first port. The input output relation can be expressed as 1->2->3->1. A pump laser is a laser or optical source used as an optical stimulating source to cause a laser transition to occur in an optical waveguide (fiber, crystal, polymer, etc) that is doped with ions that can lase. A doped fiber or waveguide is an optical fiber or optical waveguide that is doped with ions that can lase when stimulated with an optical signal of the appropriate wavelength to cause a laser transition to occur in the dopant material. An amplified spontaneous emission (ASE) source is an optical source with a broadband emission spectrum caused by the free run condition of a doped optical fiber (or waveguide) stimulated by the appropriate optical wavelength allowing super fluorescents to occur. A super luminescent diode (SLD) is a light emitting diode (LED) that has been designed to work with high current injection levels causing a spontaneous emission in a gain medium. It is typically similar in structure to laser diodes with the exception of having modified facets to eliminate or minimize a resonant (oscillation) condition. An optical isolator is a non-reciprocal optical element whose propagation properties change when the normal input and output are interchanged. An optical isolator allows the flow of optical signal in one direction only.